In preparing a planographic printing plate from a presensitized planographic printing plate (hereinafter referred to also as a PS plate), a light sensitive layer of the PS plate is marked by a ball point pen in order to register an original through which image wise exposure is carried out, and then imagewise exposed. Thereafter, the exposed PS plate is developed with an alkaline developer to remove the light sensitive layer at non-image portions to obtain a planographic printing plate. However, the ball point pen ink on the light sensitive layer at non-image portions to be removed is not removed by the developer, but remains on the support of the planographic printing plate, resulting in a problem of stain occurrence in printing. This phenomenon is called residual ball point pen ink.